In a well known temperature compensated voltage supply, the difference between two base-emitter voltages is added to a third base-emitter voltage. When the values of the components of the circuit are correctly chosen, a reference voltage results which is substantially independent of temperature. However, this voltage has a fixed value, namely the value of the bandgap voltage. It is thus not possible with this type of circuit to choose a value of the output or reference voltage.
A reference current source is described in IEEE Journal of Solid-State Circuits, Vol. SC-11, No. 6, Page 798. This includes two transistors having different emitter areas. Two identical resistors and a difference amplifier cause the two collector currents to be equal. The voltage across the base-emitter circuit of one of the transistors is inversely proportional to absolute temperature. This voltage is applied across a resistor so that a current also proportional to absolute temperature results. A second current is proportional to the base-emitter voltage of the other transistor and flows over another resistor. The sum of the two currents, when correctly proportioned, is substantially independent of temperature. However, this circuit requires a relatively high operating voltage, a relatively large amount of equipment and, finally, constitutes a closed loop circuit whose stability must be assured by additional components.